A torque converter incorporated in an automatic transmission for an automotive vehicle allows some slippages, even at cruising speeds. This is due to the fact that the only connection between pump and turbine is the transmission fluid. To prevent this slipping action, and thus improve fuel economy, a number of torque converters are equipped with a lockup feature. When a lockup unit or clutch is activated, the pump or impeller and turbine are mechanically locked together. To allow a lockup condition, most setups are arranged so that the transmission must be in high range and in a cruise condition. In a low range of vehicle speeds where the lockup torque converter is generally apt to transmit changes in engine output torque to wheels of the automotive vehicle and therefore, the automotive vehicle is uncomfortable to ride in, the lockup clutch is released so as to unlock the lockup torque converter, thereby operating as a ordinary torque converter which can multiply engine torque and absorb changes in engine output torque. On the other hand, in a high range of vehicle speeds where changes or fluctuations in engine output torque are not significant, the lockup torque converter is locked so as to prevent its slippage action and improve fuel economy.
In addition to the transmission of changes or fluctuations in engine output torque to the wheels, one of other problems is presented by increased engine torque transmission losses caused by the lockup torque converter in the low range of vehicle speeds where the lockup clutch is released so as to unlock the lockup torque converter. Another problem encountered in the lockup torque converter is that torque transmitted to the wheels changes considerably when the automatic transmission shifts while the torque converter is still locked. As a result, shocks are possibly encountered in the automotive vehicle upon the automatic transmission shifts itself. On the other hand, in addition to such shocks, the automotive vehicle is possibly subjected to a relatively large shock upon the lockup torque converter is locked immediately after the shifting of the transmission.
To eliminate these shocks, the lockup torque converter is controlled by a slip control system in such a way as to allow some slippages between the impeller and turbine when the automotive vehicle operates within a definite range of engine load and/or vehicle running speed conditions or when the automatic transmission is shifted, thereby decreasing torque losses so as to suppress or prevent vibrations of the automotive vehicle or shocks upon the automatic transmission shifts itself. To adapt the lockup torque converter to allow some slippages, the slip control system develops a predetermined working pressure to be applied to the lockup clutch so regulated as to produce a desired speed difference of rotation between the impeller and turbine. Such a slip control system is known from, for example, Japanese Unexamined Patent Publication Nos. 60(1985)-116929 and 60(1985)-14653.
In the slip control system described in the above mentioned publications, the regulated working pressure is constant, regardless of whether or not the automotive engine is slowing down such that the automatic transmission shifts itself.
Meanwhile, when the automatic transmission shifts, in particular downshifts into low range, the automotive engine should perform different operations in accordance with whether or not the engine is slowing down. In almost all of modern automotive engines, fuel cut-off in which the delivery of fuel to the engine is interrupted is effected when the automotive engine is slowing down while operating at speeds of rotation higher than a predetermined speed of rotation of, for example, approximately 1000rpm and released when the automotive engine reaches speeds of rotation lower than the predetermined speed of rotation. Accordingly, there are two somewhat conflicting requirements to such engines that govern fuel economy and the ability of acceleration. That is, when the automatic transmission downshifts into low range while the automotive engine slows down or decelerates, it is required not only to increase the effect of engine braking but also to prolong a time period for which the fuel cut-off is continued by suppressing the drop of the speed of rotation of the automotive engine for fuel economy and, on the other hand, when the automatic transmission downshifts into low range although the automotive engine is not in deceleration, it is required that the automotive engine rapidly increases its speed of rotation for desired acceleration. However, there is no slip control system that applies different working pressures to the lockup torque converter in accordance with engine operating conditions when the automatic transmission shifts, in particular downshifts into low range.